David G wrote:

I have to replace all the ceiling joists in my lounge following
ripping down the lathe & plaster and discovering that most of the
joists are considerably bowed in all different directions and won't
give me the flat finish I am after for the plasterboard.

Before you pull your ceiling down althogether, how bowed are they? Could
you pack out with loads of bits of wood to make the surface relatively
flat, then allow the plasterboard to bend a tiny bit? You can also
improve things by putting the finishing skim on thicker in some places
than others. I have done this when replacing lath and plaster with
plasterboard in my house, and nobody notices (least of all me).

Ben.